Several studies indicate that the increase of CO2 in the atmosphere has a major effect on global climate. Optimized and safe storage of CO2 is a desired and important issue in order to reduce CO2 in the atmosphere.
As an example concerning commercial storage of CO2, the Sleipner carbon capture and storage (CCS) project operated by Statoil is located 250 km off the Norwegian coast. The CO2 is stored in supercritical state in the Utsira formation at a depth of 800-1000 m below the sea surface. The CO2 produced during natural gas processing is captured and subsequently injected underground. CO2 injection started in October 1996 and by 2012, more than 13 million tons of CO2 had been injected at a rate of approximately 2700 tons per day. A shallow long-reach well is used to take the CO2 2.4 km away from the producing wells and platform area. The injection site is located beneath a local dome of the top Utsira formation.
The In Salah CCS Project is an onshore project for the production of natural gas located in the Algerian Central Sahara. The Krechba Field produces natural gas containing up to 10% of CO2 from a number of geological reservoirs. CO2 has been stripped from the gas and re-injected into a sandstone reservoir at a depth of 1800 m enabling the storage of up to 1 Mt of CO2 per year.
While the global capacity to store CO2 deep underground is believed to be large, the development of a new storage site is inevitably costly as it requires an assessment of potential risk to humans and the ecosystem. It is thus desirable that existing sites are exploited to maximum capacity. Current estimates suggest that the existing methods used to inject supercritical CO2 into deep storage sites result in only around 2% of the pore volume of the geological storage site being utilized for CO2 sequestration. This is believed to be due to the uneven sweep of the injected CO2 in subterranean formations, which leads to a phenomenon called “fingering” in which the CO2 injection front is highly uneven with small areas of high penetration surrounded by areas in which the CO2 has not penetrated at all. Pursuing current practices will result in the loss of considerable storage volume in available storage sites.
For the purpose of sequestration, CO2 is injected as a supercritical fluid into the storage reservoir, normally without designing the composition of the preferred phase of the injected stream at reservoir conditions. In WO 2012/041926, the Composition Swing Injection (CSI) technique is described, in which the composition of the injected stream is changed in cycles to create gas-like and liquid-like states in order to stabilize the CO2 plume during the injection period. This stabilisation helps to maximise the CO2 storage capacity of the reservoir.
Depending on pressure and temperature of the storage reservoir, achieving the required phase behaviour in the Composition Swing Injection (CSI) technique may require considerable modification of composition of the injected stream. One option disclosed in WO 2012/041926 is that various hydrocarbon components could be injected together with CO2 in order to obtain the required effect. In some cases these hydrocarbon components could be available from natural sources or as a by-product of oil and gas processing. However, in other cases hydrocarbons may be too valuable to use as a gas blend component, and could result in Composition Swing Injection being economically undesirable.
There remains, therefore, a need to develop a method for storing CO2 in storage reservoirs where the CSI technique is inexpensive and which overcomes the issues of loss of storage volume discussed above. It is also desirable to be able to monitor CO2 storage reservoirs for CO2 plume movement and possible CO2 seepage through the storage reservoir. Moreover, it would be advantageous to develop an economically viable method for monitoring CO2 levels. Ultimately, a method which is able to address both these problems simultaneously is desirable.
Surprisingly, the present inventors have found that methods utilising the Composition Swing Injection technique and in which the hydrocarbons are replaced by one or more CO2 soluble polymers represent an attractive solution to these issues.